The point is that I had a perfect good working postfix until I updated to version 2.10.0
I can only guess it has something to do with that, because before the update everything worked just fine.

I would like to confirm that postfix 2.9.10 after the recent update stopped relaying on the submission port (587), although prior to the upgrade it worked fine for months. I could not figure our where the problem is, so I downgraded to 2.9.5 (still working).

It may have something to do with USE flags flags or a subtle change of configuration options.